需求

某分散式系統中，主節點可以有多台，可以動態上下線，任意一臺客戶端都能實時感知到主節點伺服器的上下線。

需求分析

具體實現

先在集群上創建/servers節點

一些依賴

pom.xml：

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.mcq</groupId>
  <artifactId>zookeeper1026</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <dependencies>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>RELEASE</version>
		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>
			<artifactId>log4j-core</artifactId>
			<version>2.8.2</version>
		</dependency>
		<!-- https://mvnrepository.com/artifact/org.apache.zookeeper/zookeeper -->
		<dependency>
			<groupId>org.apache.zookeeper</groupId>
			<artifactId>zookeeper</artifactId>
			<version>3.4.10</version>
		</dependency>
</dependencies>
</project>

 在src/main/resources下添加一個file log4j.properties：

log4j.rootLogger=INFO, stdout  
log4j.appender.stdout=org.apache.log4j.ConsoleAppender  
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout  
log4j.appender.stdout.layout.ConversionPattern=%d %p [%c] - %m%n  
log4j.appender.logfile=org.apache.log4j.FileAppender  
log4j.appender.logfile.File=target/spring.log  
log4j.appender.logfile.layout=org.apache.log4j.PatternLayout  
log4j.appender.logfile.layout.ConversionPattern=%d %p [%c] - %m%n

伺服器端向 Zookeeper 註冊代碼

package com.mcq.zookeeper1026;

import java.io.IOException;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;

public class DistributeServer {
	private String connectString = "hadoop103:2181,hadoop104:2181,hadoop105:2181";
	private int sessionTimeOut = 2000;
	private ZooKeeper zk = null;
	private String parentNode = "/servers";

	public void getConnect() throws IOException {
		zk = new ZooKeeper(connectString, sessionTimeOut, new Watcher() {

			@Override
			public void process(WatchedEvent event) {
				// TODO Auto-generated method stub

			}

		});
	}

	// 註冊伺服器
	public void registServer(String hostname) throws Exception {
		System.out.println(zk);
		String path = zk.create(parentNode + "/server", hostname.getBytes(), Ids.OPEN_ACL_UNSAFE,
				CreateMode.EPHEMERAL_SEQUENTIAL);
		System.out.println(hostname + " is online " + path);
	}

	// 業務功能
	public void business(String hostname) throws InterruptedException {
		System.out.println(hostname + " is working ……");
		Thread.sleep(Long.MAX_VALUE);
	}

	public static void main(String[] args) throws Exception {
		// 1.獲取zk連接
		DistributeServer server = new DistributeServer();
		server.getConnect();
		// 2.利用zk連接註冊伺服器信息
		server.registServer(args[0]);
		// 3.啟動業務功能
		server.business(args[0]);

	}
}

客戶端代碼

package com.mcq.zookeeper1026;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;

public class DistributeClient {
	public static void main(String[] args) throws IOException, KeeperException, InterruptedException {
		//1.獲取zk連接
		DistributeClient client=new DistributeClient();
		client.getConnect();
		//2.獲取servers的子節點信息，從中獲取伺服器信息列表
		client.getServerList();
		//3.業務進程啟動
		client.business();
	}

	private void business() throws InterruptedException {
		// TODO Auto-generated method stub
		System.out.println("client is working ……");
		Thread.sleep(Long.MAX_VALUE);
	}

	private String parentNode="/servers";

	private void getServerList() throws KeeperException, InterruptedException {
		// TODO Auto-generated method stub
		//1.獲取伺服器子節點信息，並且對父節點進行監聽
		List<String> children = zk.getChildren(parentNode,true);
		//2.存儲伺服器信息列表
		ArrayList<String> servers=new ArrayList<>();
		//3.遍歷所有節點，獲取節點中的主機名稱信息
		for(String child:children) {
			byte[] data=zk.getData(parentNode+"/"+child,false,null);
			servers.add(new String(data));
		}
		//4.列印伺服器列表信息
		System.out.println(servers);
	}

	private ZooKeeper zk=null;
	private String connectString="hadoop103:2181,hadoop104:2181,hadoop105:2181";
	private int sessionTimeOut=2000;

	private void getConnect() throws IOException {
		// TODO Auto-generated method stub
		//創建到zk的客戶端連接
		zk=new ZooKeeper(connectString,sessionTimeOut,new Watcher() {

			@Override
			public void process(WatchedEvent event) {
				// TODO Auto-generated method stub
				try {
					getServerList();
				}catch(Exception e) {
					e.printStackTrace();
				}
			}
			
		});
	}
}

運行程式

先運行客戶端

在xshell上登錄客戶端（bin/zkCli.sh）,進行一些操作，可以看到eclipse的控制台會實時輸出有哪些節點。

然後運行伺服器端，記得先在run configuration里設置一下參數，比如設成hadoop103，那麼運行後，客戶端的控制台會實時顯示當前的節點。

 點上面那個紅框可以切換控制台。